Hot water tanks with a small capacity, that is, particularly for household purposes, have been provided heretofore practically exclusively with a passive anticorrosive layer, mostly of enamel, as well as with a donor anode of magnesium. The magnesium anode is screwed from the top through a socket into the tank while the electric heating element is arranged above a flange in the lower part. It was found in practice that these hot water tanks are destroyed by corrosion after a few years, particularly in the lower region receiving the heat element. Thorough investigations by means of potential and current measurements showed that in these hot water tanks, the lower third is not cathodically protected is the heating flange is insulated from the boiler. The heating flange is usually not insulated for electrotechnical reasons.
The use of magnesium donor anodes has other disadvantages, in particular, magnesium anodes show considerable natural corrosion, so that they are often completely destroyed or used up after a few years. Furthermore, a considerble amount of hydrogen gas is formed on a magnesium anode which forms an oxyhydrogen gas with oxygen, which is dangerous.
For larger tanks, it has therefore been suggested to use, instead of magnesium donor anodes, electrodes supplied with external current, particularly of aluminum, ("Handbuch des kathodischen Korroisionschutzes" by Raeckmann-Schwenk, Publ. by Chemie GmbH, 1974, chapter XVI, p. 330 and 331). This did not lead, however, to a satisfactory result because the electrodes supplied with external current are always set to a single current level. Accordingly, when the conditions in the tank change, e.g., when an additional defective spot appears in the passive anticorrosive layer, the correct potential on the tank wall will no longer be set.
Besides, the present electrodes supplied with external current do not provide a solution for the problem of the special corrosion hazard in the range of the heating element of a water tank equipped with such an element. The damage in this region results from the surfaces of the heating element acting as a cathodic region in which the oxygen is reduced and can act as an oxidant. Accordingly, any defects in this region are particularly subject to corrosion in the passive anticorrosive layer.
It is also known in large objects, e.g. pipe line systems and ships ("Handbuch des kathodischen Korrosionsschutzes" chapter VI, p. 151-153), to use potentiostatically regulating d-c sources in cathodic corrosion protection within the protective systems for external current. This is done in such a way that the protective current is automatically adapted to the normal situation under changing conditions, e.g., in the pipe lines in the radius of action of d-c paths, or with the appearance of a defective spot in the passive anticorrosive layer of a ship's hull.